Ken Livingstone today launched his bid for his "last political role" – a comeback as the mayor of London.

The Labour veteran, who was ousted by the flamboyant Tory Boris Johnson in 2008 after eight years in City Hall, officially opened his campaign to be the Labour candidate in 2012 by attacking his successor over steep rises in public transport fares.

If selected to be the Labour candidate, Livingstone, who indicated that he had intended to make a comeback bid early last year, would be entering his fifth decade in political life.

He said his priority would be to "protect" ordinary Londoners from the recession and government spending cuts, and promised to freeze his own and senior advisers' pay for the full four-year term.

In a nod to critics who accused him of having focused on inner London as mayor at the expense of the rest of the capital, Livingstone launched his campaign in a shopping centre in Croydon, outer London.

He turned his fire on the incumbent mayor and released a list of high-profile Labour politicians from across the capital backing his campaign.

"I want to be mayor for one overriding reason: if I am elected, my focus will be to do everything I can to protect Londoners from the recession and the effects of the government's policies," he said.

"The global economic crisis, a fragile recovery that may go double-dip, and a government removing billions from the economy and planning cuts on a scale that Britain has not seen for decades, mean the mayor's priority must be to protect Londoners.

"I will use every lever to make sure our quality of life is protected and improved. Not a mayor who spends his time defending bankers – instead, one who will use mayoral budgets and powers to protect ordinary Londoners."

He said his top priority would be transport fares, which saw above-inflation rises in January to plug what Johnson described as a "massive black hole" in the finances of Transport for London, which the capital's mayor chairs.

Livingstone added: "Under Boris Johnson, a single bus fare by Oyster [the smart card travel payment system] has risen by a staggering one-third, as has the price of a weekly bus pass.

"Instead of wasteful projects, the obsession with academy schools, the zany floating airport in the Thames Estuary, we must concentrate on defending public services and holding down fares."

The mayoral hopeful also vowed to tackle "scandal" of the lack of affordable housing in the capital, which results in long waiting lists.

He intends to press the government for powers to raise money on the bond markets to build affordable homes, including for rent, and cut energy bills by improving insulation in every building in London over 10 years.

He entered the contest days after the ex-MP Oona King announced her intention to fight him for the nomination and promised "a breath of fresh air" for the capital.

Livingstone said he and King presented a "straightforward left/right contest", citing as an example King's support for the 2003 Iraq invasion – expected to play badly among some of the 30,000 Labour members in London who will have a say in the candidate selection process.

Livingstone, a vocal opponent of the invasion, said he would not make Iraq a feature of the contest against King unless "she wants to get into negative campaigning".

The 64-year-old has been angered by thinly-veiled attacks on his age by King at her campaign launch last week.

Referring to the choice facing voters at the last mayoral election as "between a man with blond hair and one with grey hair", King said Londoners did not need a "popularity contest based on who's wacky or who's stale".

Livingstone said: "It is not a question of age. It is a question of politics. She stands on the right of the party and I stand on the left. I don't think my hair colour is a relevant feature. It if was, I would start dyeing my hair."

He insisted few candidates would put their hat in the ring because he is standing, adding: "I am sure if I dropped dead lots more people would run.

"A lot of people in politics do not want to stand unless they are certain of winning. My view is that the worst thing that can happen is that you do not win. I don't see that as a personal rejection, just that someone is better or it's a bad time."

Livingstone is hoping for a rematch with Johnson, who beat him after securing 53.2% of votes in 2008, although the latter has kept his options open about a standing for a second term.

The former mayor is a regular attendee at the monthly City Hall question time in which Johnson is put in the hotseat.

He blames the double whammy of Labour's unpopularity at the last mayoral election and critical coverage of his mayoralty in the run up to the 2008 election by the Evening Standard for his defeat.

"I could not think of a worst background for an election," he said, pointing out that 200,000 more Londoners voted for him in 2008 than at the previous poll in 2004.

Livingstone quipped that his only regret as mayor was not to close down the Standard a year before the 2008 election.

But in a sign of rapprochement with his old foe, now under different management, Livingstone set out his stall in the London paper beneath the headline "Comeback Ken".

"I will use every lever to make sure our quality of life is protected and improved," he wrote. "Not a mayor who spends his time defending bankers – instead, one who will use mayoral budgets and powers to protect ordinary Londoners.

"I would overhaul London's budget priorities in favour of Londoners as a whole, not hedge fund managers or polluters."

Labour's decision to run the selection timetable for the mayoralty almost parallel to the party leadership contest will give the candidate at least 18 months to campaign and shadow Johnson.

Hopefuls have until 18 June to put their names forward, and will then be whittled down to a shortlist by a panel of national and London party representatives on 24 June before a series of hustings across the capital.

An electoral college, made up half of votes by London party members and half by members of affiliated organisations, will then pick the candidate.

The result will be announced on 24 September – the day before Labour reveals who has won the race to succeed Gordon Brown as the party leader.